Carburizing steel articles



Patented Dec. 27, 1949 CARBURIZING STEEL ARTICLES.

'Banl-;M.. Leininger, Grand Island, N. Y.,;assjgno r to E. I. du Pont de Nemours & Cmpany,.Wil- ,mington, :Del., ,a. corporation of Delaware No Drawing. Application March-23,1946,

Serial No. 656,765

Claims. 1 This invention relates to carburizing steel articles and more particularly to treating steel articlesin a molten salt-bath so as-to form-a carburized case, thereon,

In e har nin pe ations-mol en. sal ba containing cyanide have been utilized to car.- burize steel articles, "but such baths invariably introduce notv only carbon into the steel, butalso more or less nitrogen. For some purposes, the introduction of nitrogen is objectionable and many attempts have been made to develop oommercially suitable molten salt baths which will carburize with little or no nitriding. The most successful of such baths contain cyanide or other carbon-nitrogen compounds plus other ingredients which accelerate carburizing or repress nitriding, but such baths are never entirely devoid of nitriding action.

Attempts also have been made to develop carburizing baths which contain no nitrogen compounds, so that nitriding is completely prevented. For this purpose, because of its relative cheapness, uncombined carbon is the preferred carburizing ingredient and several proposals for carbon-containing carburizing baths have been made. However, such baths usually have required undesirably high operating temperatures or have had undesirably low carburizing activity.

An object of the present invention is to provide an improved carburizing process utilizing a molten salt bath free from nitrogenous carburizing ingredients. Another object is an improved carburizing bath and process utilizing carbon as the carburizing ingredient and operable at conventional bath carburizing temperatures. Another object is to increase the carburizing activity of carbon in a molten salt bath. Still other objects will be apparent from the ensuing description of the invention.

The aforesaid objects are attained in accordance with the present invention by treating steel articles to be carburized in a molten salt bath containing one or more alkali metal carbonates, at least 2% by weight of finely divided carbon dispersed in the bath and from 1 to about by weight of an oxygen compound of boron or silicon which is soluble in the bath, at a temperature of about 1500 to 1800 F. The preferred addition agent is boron oxide (B203).

In place of boron oxide (B203), other oxygen compounds of boron and silicon which are more acidic than sodium carbonate, may be employed as the activating agent. Sodium tetraborate, boric acid, silica and alkali metal silicates are typical examples. The quantity of the activating agent required will vary depending upon the conditions employed, but generally will lie-in the range of 1% and.2 %,.by weight.

The amount, of-carbonatepresent.is not critical, as long as an excess of .at,least about:5% above that required tQ-reactwiththe.addedboron oxide or other activating; a gent, iswmaintained, i. e., about 5% in excess-over {one :molar equivalent.

The amount of carbon dispersed inv thebatn will depend on the nature. of thecarbon employed, its-state of subdiyision, ,andthebathcomposition. Generally, the. bath. should contain about 2 to 15% by weight of dispersed carbon. It is preferable to employ an amount of carbon in excess of that which can be dispersed, by adding suflicient to form a floating layer of carbon on the surface of the bath. The various known kinds of carbon can be used, c. g., graphite, charcoal, activated carbon and the like.

The steel articles to be carburized are treated by immersion in the bath for a time dependent on the desired depth of case, e. g., for from 10 minutes to 48 hours.

Since the carburizing action falls off gradually following the addition of the boric acid, it will be necessary in those cases where continuous operation is desirable to replenish the bath periodically with fresh carbonate and boron oxide or its equivalent, while maintaining a sufficient excess of carbon in the melt.

Following is a specific example of the process:

Example A 50% sodium carbonate-50% potassium chloride bath containing excess carbon was operated at 1600 F. An SAE X1020 steel bar treated therein for 1 hour showed surface decarburization (loss of carbon) to a depth of about 0.014". Five per cent of the bath weight of boric acid anhydride (B203) was added and a second SAE X1020 steel bar was treated in the bath for 1 hour. With this short time a nitrogen-free carburized case 0012-0016 in depth was obtained. Following are the analyses of gradient cuts removed from the surface of the steel test bars employed. Each cut was 0.004" on radius.

Per Cent 0 Per Cent 0 Out No. Before Add- After Add- 0. 08 0. 42 0. l6 0. 36 0. l8 0. 28 0. 20 O. 24 1 0. 22 0. 22

3 The specifications of SAE X1020 steel are:

0.15-0.25% carbon 0.70-1.00% manganese 0.045% phosphorus maximum 0.055% sulfur maximum I claim:

1. The process which comprises immersing steel articles in a molten salt bath consisting of at least one alkali metal chloride, together with about 2 to 15% by weight of finely divided carbon dispersed in the bath, from 1 to 20% by weight of an oxygen compound more acidic than alkali metal carbonate selected from the group consisting of the oxygen-containing compounds of boron and silicon which are soluble in said bath and an amount of alkali metal carbonate not less than about 5% more than one molar equivalent of said oxygen compound, at a temperature of 1500 to 1800 F.

2. The process of claim 1 in which said oxygen compound is a boron compound.

3. The process of claim 1 in which said oxygen compound is boron oxide.

4. The process which comprises immersing steel articles in a molten salt bath consisting of approximately equal parts by weight of sodium carbonate and potassium chloride, together with about 2 to of finely divided carbon dispersed in the bath and 1 to by weight of boron oxide, at a temperature of 1500 to 1800 F.

5. The process which comprises immersing steel articles in a molten salt bath consisting of a mixture of alkali metal carbonate and alkali metal chloride containing about 2 to 15% by weight of finely divided carbon dispersed in the bath and 1 to 20% by weight of boron oxide, the amount of said carbonate being not less than 5% more than one molar equivalent of the boron oxide present and not more than about of the combined weights of said carbonate and said chloride.

PAUL M. LEININGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,796,248 Freudenberg Mar. 10, 1931 2,063,079 Beck Dec. 8, 1936 2,095,188 Hanusch Oct. 5, 1937 2,364,292 Holt Dec. 5, 1944 OTHER REFERENCES Steel and its Heat Treatment, Bullens, vol. 1, 4th edition, 1938, John Wiley & Sons Inc., N. Y., page 344.

The Iron Age," November 10, 1932, page 16, and December 8, 1932, page 882. 

